The present invention relates generally to an improved method for producing alloy of low thermal expansion, and more particularly to an improvement in production of an Fe-base alloy well suited for production of lead frames used for semiconductor integration circuits.
Fe-Ni type 42 alloy, 54Fe-29Ni-17Co type Fernico alloy and Fe-Ni-Co type alloy disclosed in Japanese Patent Opening No. sho.59-198741 are well known as alloys of low thermal expansion.
The Fe-Ni type 42 alloy has a low magnetic transformation point in a range from 300.degree. to 350.degree. and exhibits extremely large elongation by thermal expansion in the temperature range above the magnetic transformation point. This elongation is by far larger than that of Si forming silicon chips, and when the lead frame of a semiconductor integration circuit is made of the Fe-Ni type 42 alloy, such a big difference in elongation imposes thermal stress on silicon chips in the integration circuit due to thermal expansion and shrinkage of the lead frames at excitation and deexcitation of the integration circuit.
Although the 54Fe-29Ni-17Co type Fernico alloy has a relatively high magnetic transformation point, its elongation by thermal expansion is significantly larger than of Si in the temperature range below the magnetic transformation point. When the lead frame of a semiconductor integration circuit is made of the 54Fe-29Ni-17Co type Fernico alloy, such a big difference in elongation also imposes thermal stress on silicon chips in the temperature range below the magnetic transformation point at excitation of the integration circuit.
The above-described Fe-NiCo type alloy exhibits elongation by thermal expansion closer to that of Si than the Fe-Ni type 42 alloy and the 54Fe-29Ni17Co type Fernico alloy in the high temperature range, and thermal stress imposed the silicon chips in the integration circuit is extremely small. Nevertheless, the alloy undergoes transformation in the temperature range below the room temperature and its thermal expansion varies greatly in the low temperature range. As a consequence, use of this alloy also imposes thermal stress on the silicon chips in the integration circuit in the low temperature range.